Snow Cover Duration

About the map

How long does snow cover last each year and has that behavior been changing? This interactive map shows monthly snow cover anomalies for the Northern Hemisphere. A different map display is available for every month of every year in the time series.

Snow cover is measured in duration, meaning how many days a particular place was covered by snow, and whether the number of snow-covered days was above or below the long-term average for the period of 1966 to 2015.

This interactive map is derived from visual analysis of early satellite data combined with more recent satellite observations. It provides an unbroken record of snow cover extending back well before the start of the continuous polar-orbiting satellite record (in 1978). Because it incorporates older technology, the data record comes with the tradeoff of coarse resolution. Snow cover is shown in large cells, and depending on the location, an individual cell may range in size from 100 to 200 kilometers on each side. 

The corresponding bar graph shows how much the overall snow cover extent for that month departs from the long-term average.

Color key and bar graph

When you select a month on the dropdown selector and a year on the lower-left slider, the map will show snow cover.

• Snow cover anomalies of higher-than average snow cover duration will appear in shades of blue, and snow cover anomalies of lower-than-average snow cover duration will appear in shades of orange and red.
• Because these maps describe snow cover duration for the entire Northern Hemisphere, data cells will spill over the bounds of the Arctic.
• Areas with snow cover duration equal to the long-term monthly average are white. This includes land areas where the average snow cover for the time period is zero, i.e., are snow free.
• The greater the snow cover duration departure from average, the darker the color.

The bar graph indicates the anomaly (departure from the long-term average) in snow cover extent for the entire Northern Hemisphere, including areas not visible in the map. Blue bars show positive anomalies (years in which snow cover extent is greater than the long-term average) and red bars indicate negative anomalies (years in which snow cover extent is less than the long-term average). The bars show every year in the time series for the selected month, and the bar that correlates with the map on display is highlighted in light gray.

How to change the display

• To change the month displayed on the map and graph, use the dropdown “month” selector in the bar graph box (lower right). 
• To change the year displayed, move the slider in the year box (lower left).
• To animate the time series for the selected month, click the play arrow. The animation will display maps for the selected month for all years in the time series.

What's so important about snow?

Depending on your perspective, snow can be a source of fun snowball fights and champagne-powder ski slopes, a source of unwelcome shoveling obligations and stressful commutes, or a distant phenomenon in colder climates. Love it, hate it, or rarely think about it, snow plays an important role in the landscapes and ecosystems we know. It even affects the whole planet’s climate system.

Locally and regionally, snow limits wild animals' wintertime access to forage, influencing migration and hibernation. Snow acts as an efficient insulator that affects the temperature of tundra soils. Seasonal snow cover also acts as a natural water reservoir, storing solid precipitation in winter and releasing it during the spring and summer melt. In the Arctic, snow's storage-and-release capacity drives the discharge regimes of Arctic-draining rivers. And across the world, more than a billion people rely on snow as a resource for drinking water, irrigation, and/or hydroelectric power. Significant changes in how long snow persists on the ground can change the timing and amount of available water for communities dependent on snowmelt.

Snow also has global climate implications. Snow is the most reflective of all the natural materials occurring on Earth's surface. Snow typically reflects between 50 and 90 percent of the solar radiation that reaches it which creates a cooling effect. As a result, snow cover plays a key role in regulating Earth's climate system.

Although snow cover occurs in some parts of the Southern Hemisphere, Northern Hemisphere snow cover plays a more significant role in Earth's climate. The Northern Hemisphere holds more land area than the Southern Hemisphere, and much of the Northern Hemisphere land is at a high enough latitude to host snow cover. The annual expansion and contraction of snow cover in the Northern Hemisphere winter is one of the biggest recurring patterns in the planet's reflectivity of sunlight.

When much of Earth's land area is snowy white, more energy is reflected back into space keeping temperatures cooler. In other words, plentiful snow cover can increase the odds that additional snowfall will persist.

Decline in snow cover—whether in the amount of land covered by snow or the amount of time snow cover persists—might contribute to Arctic amplification. Arctic amplification is defined as the more rapid warming of the Arctic compared to the rest of the globe. If warmer temperatures melt snow sooner, there is less snow cover to reflect the Sun’s energy and cool the planet. Less snow cover exposes soil and vegetation, creating darker-colored regions that absorb more of the Sun's energy, which in turn warms the ground, causing more snow cover to melt. In other words, shorter durations of snow cover can create sparser snow cover which can in turn increase the odds that subsequent snowfall will melt quickly.

What the data show

The climate data record used for this interactive map starts in late 1966. At that time, snow maps were drawn by hand based on early satellite observations. In the early 1970s, improvements in satellite technology provided increasingly reliable data. Starting in the late 1990s, further technological improvements replaced weekly maps with daily maps at higher resolution. Snow cover data have periodically been reanalyzed to produce a consistent record dating back to the 1960s.

These snow cover extent maps show that, in the winter months, Arctic snow cover duration is more consistent. Much of the Arctic is so consistently cold that precipitation nearly always falls as snow and rarely melts right away. During the rest of the year, areas of above-average and below-average Arctic snow cover duration appear annually from May to September. At lower latitudes in the Northern Hemisphere, both positive and negative anomalies can be seen during the winter months, reflecting variability of the passage of winter storms over the North American and Eurasian continents as well as widespread melt events.

Despite annual variability, snow cover extent has declined across the Arctic. According to the Arctic Report Card: Update for 2020, over the period 1981 to 2020, May snow cover extent declined by 3.7 ± 2.0 percent per decade. Over the same period, June snow cover extent declined by 15.5 ± 6.1 percent per decade. Meanwhile, the Rutgers University Global Snow Lab reports that snow cover extent has declined across the Northern Hemisphere since the mid-1960s, although with substantial year-to-year variability.

By viewing different months and years, you can use this map to examine changes in snow cover duration and extent over time, and where those anomalies happen most often. Try using these maps to answer questions such as:

• In what years do declines in snow cover duration become common?
• In which months are anomalies biggest?
• In which months are anomalies smallest?
• Have snow cover duration anomalies—snow lasting longer or melting sooner than average—become more common in certain regions inside or outside the Arctic?

You can compare snow cover maps with other maps in Satellite Observations of Arctic Change, such as maps for near-surface air temperature and non-frozen ground.

The data shown here are from the NOAA Climate Data Record (CDR) produced by the Rutgers University Global Snow Lab. 

Data source(s)

The climate data record used in this interactive map is based on satellite-derived maps of snow cover produced by NOAA. This record of snow cover is the longest continuous environmental remote sensing data set. Snow maps were initially produced on a weekly basis. Since the late 1990s, snow maps have been produced daily. The Global Snow Lab has gone to great efforts to make the record of snow cover consistent through time. The CDR consists of weekly snow cover maps showing a pixel as snow covered or snow free. More information is available from Rutgers University Global Snow Lab.

Explore the source data for this map:

Global Snow Lab. Rutgers University. Accessed April 12, 2021.

Data processing steps

To create this map, NSIDC took the following steps:

• Download Northern Hemisphere snow cover area CSV from Rutgers.
• Download Northern Hemisphere weekly snow cover dataset from NCDC: ftp://data.ncdc.noaa.gov/cdr/snowcover/nhsce_v01r01_19661004_latest.nc (You may need to use a File Transfer Program to download this file.)
• Compute monthly snow cover extent anomaly CSV. Compute the climatological mean snow cover extent for each month across 1966-2015. Subtract the monthly climatological mean from each year/month in the timeseries.
• [a] Compute the monthly average snow cover grids. For each year/month in the timeseries:
  • Find the weekly extent grids that ‘touch’ that month. Compute a weighted average extent using those weekly grids.
• [b] From the monthly grids [a], compute the climatological mean snow cover grids (1966-2015)
• From the monthly grids [a] and climatological means [b], create monthly images
• Images are in polar stereographic projection (PROJ4 string: '+proj=stere +lat_ts=60 +lat_0=90 +lon_0=-80 +a=6378137 +b=6356752.3142 ')

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